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A couple of times I've tried to say "I agree with..." or "he agrees with..." but I don't know whether I'm using 同意{どうい}する correctly.

In what ways should に同意{どうい}している be used? Also, which particles should be used? Can only に be used before 同意{どうい}する or are there circumstances where other particles are used as well, such as と and を?

In many examples I've found using 同意{どうい}する, I've seen （だ）ということに or just （な）ことに (the だ/な after na adjectives/nouns and without だ/な after i adjectives/verbs it seems.) Is there a specific reason for using one over the other, e.g. is the longer （だ）ということに more polite, and what do these mean?

"ということ" literally means "the so-called", which I think is a little more polite.

Also to @sawa , search int the above site with "いという" and "いだという" (I know they are not words nor phrases ...) gives interesting results. It seems that wile i adjectives, "美味しいだということ" can be used.

And I think "オレンジはおいしいということ彼かれに同意どういします" is still a little wired, maybe it should be "オレンジ*が*おいしいだということ*について*彼かれに同意どういします。". A phrase with nothing in the end doesn't sound natural. (About "は" and "が", I just feel this way, I am not able to explain it in details.)